With the recent interest in fuel cells, a great deal of research into reactors for uniform heating to create hydrogen from hydrocarbons is being conducted. Hydrogen may be variously prepared using hydrocarbons, as shown in Reactions 1 to 3 below. Of these reactions, a steam reforming reaction shown in Reaction 1 is receiving researchers' attention, because the hydrogen product has a very high concentration. In addition, partial oxidation (Reaction 3) having rapid response properties is also receiving researcher's attention.CH4+H2O→CO+3H2,H2980=+206 kJ/mol  Reaction 1CO2+CH4→2CO+2H2,H2980=+247 kJ/mol  Reaction 2CH4+½O2→CO+2H2,H2980=−36 kJ/mol  Reaction 3CH4+2O2→CO2+2H2O, H2980=−801 kJ/mol  Reaction 4
As a hydrogen preparation system for a small stationary fuel cell Residential Power Generator (RPG), the steam reforming reaction, which results in a high hydrogen concentration, is expected to have more usefulness than the rapid response reaction, and thus has been intensively and thoroughly studied.
However, the reaction, which is apparent from Reaction 1, has a problem because heat required for the above reaction must be supplied. In the steam reforming reaction, when the reaction temperature is at least 750° C., the conversion efficiency of hydrocarbon (methane) can reach 95% or more. However, much effort is required to supply the reaction heat while maintaining high temperatures.
The reaction heat is produced through the combustion (catalytic oxidation or combustion) of hydrocarbon, as shown in Reaction 4. As such, in order to effectively realize heat transfer, there is a need for a material having a high temperature difference (ΔT), a large contact area (A), and a high heat transfer coefficient (k).
However, it is impossible to indefinitely increase the temperature of a flame required for heating to obtain the temperature difference. In this case, the constituent material may become degraded, and nitrogen oxide (NOx) pollutants may be generated. The heat transfer coefficient is also limited to the inherent value of the constituent material.
Thus, the reactor should be structured such that the heat transfer area is as large as possible.
To this end, attempts have been made to use a reactor comprising thin metal plates in which microchannels are formed. A plurality of thin metal plates, each of which is processed to have microchannels, is layered to obtain a large contact area per unit volume (see Korean Patent Laid-open Publication No. 2003-0091280 incorporated herein by reference).
However, since the combustion of hydrocarbon (LNG, LPG, alcohol) necessary for the production of reaction heat is an intense reaction, which produces very large amounts of heat, it may be performed through catalytic combustion or non-catalytic combustion.
In addition, a mixture comprising hydrocarbon and air may be oxidized through spontaneous combustion at a predetermined minimum temperature (e.g., 650° C. in the case of carbon monoxide, which is present in air).
Therefore, even with the use of a reactor having microchannels, if the combustion heat is partially produced at the location where the reaction material is supplied, the temperature of the reactor is non-uniform, thus creating non-uniform combustion catalyst or hot portions, resulting in decreased activity of the reforming catalyst.